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Rationale: BaSnO3 is an interesting technical and industrial ceramic, with uses in many areas of electronic technology. Currently, there is great interest in this ceramic material because of its potential as a transparent conductive oxide. Due to its good chemical stability, it is also used as a surface processing material in the synthesis of electroluminophores. When heated, the stannates of alkaline earth metals can pass into the vapor phase with or without dissociation. Until the present investigation, gaseous salts where SnO plays the role of an anion‐forming oxide had been unknown. The formation enthalpy of gaseous Ba2O2 also needed to be determined. Methods: Knudsen effusion mass spectrometry was used to determine the partial pressures of vapor species, equilibrium constants and enthalpies of the studied gas‐phase reactions, as well as the formation and atomization enthalpies of gaseous BaSnO2 and Ba2O2: a mixture of BaO and SnO2 was evaporated from a platinum effusion cell. For the evaporation of gold (pressure standard), a molybdenum effusion cell was used. A theoretical study of gaseous BaSnO2 and Ba2O2 was performed by several quantum chemistry methods. Results: Ba, BaO, Ba2O2, SnO and BaSnO2 were found to be the main species in the vapor over the BaO–SnO2 mixture in the temperature range of 1680–1920 K. The standard formation enthalpies of gaseous BaSnO2 and Ba2O2 were determined on the basis of the equilibrium constants of the studied gas‐phase reactions. Energetically favorable structures of these gaseous species were found and vibrational frequencies were evaluated in the harmonic approximation. The formation enthalpy of gaseous Ba2O2 was clarified; in addition, the formation enthalpies of gaseous SrSnO3 and CaSnO3 were estimated. Conclusions: The thermal stability of gaseous BaSnO2 was confirmed by Knudsen effusion mass spectrometry. The reaction enthalpies of gaseous BaSnO2 from gaseous barium and tin oxides were theoretically evaluated and the obtained values were found to be in reasonable agreement with the experimental ones. [ABSTRACT FROM AUTHOR]